Solid-state imaging devices with higher resolution are used in many commercial applications, especially cameras, and also for other light imaging uses. Such imaging devices typically are based on CCD (charge coupled device) or complementary metal oxide semiconductor (CMOS) image sensor with associated switching elements, and address (scan) and read out (data) lines. These CCD and CMOS image sensor technologies have matured so much that currently millions of pixels and surrounding circuitry can be fabricated using silicon based CMOS) technology. As today's CCD and CMOS image sensor technologies are based on silicon (Si), the detectable spectral ranges of CCD and CMOS sensor are limited to the wavelengths below 1 μm, where Si exhibits absorption. Additionally, CCD and CMOS image sensor-based imaging have other shortcomings, since it lacks high efficiency response combined with high quantum efficiency over broad spectral ranges. This broad spectral detection is required in many applications. One of them is the free space laser communication, where shorter (in visible ranges) and near infrared wavelengths are used. Image sensors having broad spectral detection capabilities, disclosed in this invention, are expected to provide those features not available in today's CCD, CMOS image sensor, and other imaging technologies. With a well-designed array, appreciably better resolution can be achieved.
Detectors (also known as photodiode or sensor pixel), especially of p-i-n type, have been studied extensively over the last decade for their application in optical communication. Currently, multiple wavelength ranges can be detected, but only in separate sensor for each wavelength-band (i.e., ranges from specific wavelength to other specific wavelength). Those photodiodes which have been most extensively studied are for near infrared detection, especially in the wavelength vicinity 1310 nm to 1550 nm, where today's optical communication is dealt with. Today the photodetector speed as high as 40 Gb/s, as described in the publication by Dutta et. Al. in IEEE Journal of Lightwave Technology, vol. 20, pp. 2229-2238 (2002), is also available for optical communication. These photodiodes use InGaAs material as absorption material, and the diode is fabricated on the InP wafer. On the other hand, Si substrate is used for the photodiode for detection of visible radiation. Other materials such as PbS, InAs, InSb, GaSb, PtSi, and HgCdTe have been used for detectors for wavelength-band with wavelengths greater than 1.65 μm, but they generally have to be cooled to low temperatures, often have very slow responses, or have high dark current.
For mid-wave infrared detectors (MWIR, approximately 3-μm, or 5-8 μm), the most common materials are either InSb or GaSb. Additionally, there has been some success in using type II materials in a superlattice structure for achieving wavelength-band covering MWIR. Some of the problems associated with these materials can be solved with avalanche photodiode structures, but that solution is imperfect due to the high manufacturing cost, the slower response times, and the fact that in order to decrease dark current, conversion efficiency often must be sacrificed.
For long wave infrared detection (LWIR, 8-12 μm), generally HgCdTe is used. HgCdTe is a particularly attractive material because its band gap is very flexible depending on the percentage of Hg versus Cd. Advancement in this material field, however, has been slow due to the high lattice mismatch between HgCdTe and available cost-effective substrate (e.g. Si). There has also been some success with use of type II material superlattice structure grown on InSb.
While current technology provides spectral detection in a large number of wavelength-bands r, no current technology can provide broad spectral detection capability ranging all the way from UV to long wave infrared wavelengths in a single photodetector. It is highly desirable to design the sensor having broader spectral detection ranges and can be fabricated on a single wafer. In addition, it is also important to have a single image sensor whose wavelength-band can be selectable. For covering multiple spectral ranges (a.k.a. bands), two photodiodes fabricated from Si and InP, discretely integrated, can be used. Monolithically, wafer bonding technology to bond Si and InP can be used to fabricate the photodiode covering the wavelengths from visible to near infrared. However, the reliability of wafer bonding over wide range of temperatures is still an unsolved issue and a high-speed operation is not feasible with a wafer bonding approach. It is highly desirable to have a monolithic photodetector array (forming the image sensor), which could offer high bandwidth (GHz and above) combined with high quantum efficiency over a broad spectral range (<0.2 μm to >40 μm). For use especially in imaging where CCD or Si-CMOS based image sensors are currently used, the multicolor image sensor array with the possibility to rapidly and randomly address any pixel could able to provide multiple spectral bands image and their fused image which are very much essential in numerous applications such as bio-medical, security, agriculture, communication, etc.
It is our objective to develop a monolithic photodiode and their array for broad spectral ranges covering from UV to long wave infrared wavelengths, while having high frequency response and high quantum efficiency over the entire wavelength region.
Our innovative approach utilizes surface incident type (either top or bottom illuminated) photodiode structure having a single set of absorption layers, which can provide broad spectral response due to the material used and their unique structure. The photodiode can be used as a single element and also in an array.
According to the current invention, photodiodes having ultra-broad spectral bands, from near UV to LWIR. High quantum efficiency, and high frequency response can be fabricated using a single wafer. According to this invention, in the case of a photodiode array, each photodiode can also be operated independently. Some applications include imaging applications such as for astronomical observation, communication, biomedical, security, etc.